In this work, the GaN p-MISFET with LPCVD-SiN_x is studied as a gate dielectric to improve device performance. By changing the Si/N stoichiometry of SiN_x, it is found that the channel hole mobility can be effectively enhanced with Si-rich SiN_x gate dielectric, which leads to a respectably improved drive current of GaN p-FET. The record high channel mobility of 19.4 cm²/(V?s) was achieved in the device featuring an Enhancement-mode channel. Benefiting from the significantly improved channel mobility, the fabricated E-mode GaN p-MISFET is capable of delivering a decent-high current of 1.6 mA/mm, while simultaneously featuring a negative threshold-voltage (V_TH) of –2.3 V (defining at a stringent criteria of 10 μA/mm). The device also exhibits a well pinch-off at 0 V with low leakage current of 1 nA/mm. This suggests that a decent E-mode operation of the fabricated p-FET is obtained. In addition, the V_TH shows excellent stability, while the threshold-voltage hysteresis ΔV_TH is as small as 0.1 V for a gate voltage swing up to –10 V, which is among the best results reported in the literature. The results indicate that optimizing the Si/N stoichiometry of LPCVD-SiN_x is a promising approach to improve the device performance of GaN p-MISFET.